Add support for bucket types

Riak 2 feature: Adds bucket type support 

- All `RiakBucket` instances now have an associated `BucketType`, even it's just "default".
- You can get instances off the client object using the `bucket_type()` method.
- You can fetch and set bucket-type properties over both transports.
- You can list/stream buckets under a bucket-type.